Insulin resistance is accompanied by hyperinsulinemia and activation of the renin-angiotensin system, both of which are associated with hypertension. Because the kidney plays a major role in the regulation of blood pressure, we studied the regulation of insulin receptor expression in the kidney during states of insulin resistance. Using two rat models of insulin resistance, Western blot analysis demonstrated a significant reduction in the expression of insulin receptor subunits in the kidney compared to lean control rats. Treatment of insulin resistance in Zucker rats with the insulin-sensitizing drug rosiglitazone partially restored renal insulin receptor levels. Conversely, treatment with the angiotensin II type 1 receptor (AT1) antagonist candesartan increased renal insulin receptor expression compared to untreated rats. Streptozotocin-induced hyperglycemia, which results from hypoinsulinemia, reduced expression of renal insulin receptors. Hyperinsulinemia induced by insulin infusion, however, did not produce a similar effect. In conclusion, insulin receptors are downregulated in the kidneys of insulin resistant rats, possibly mediated by hyperglycemia and angiotensin II.

Insulin has been shown to have antinatriuretic actions in humans and animal models. Moreover, endogenous hyperinsulinemia and insulin infusion have been correlated to increased blood pressure in some models. In this review, we present the current state of understanding with regard to the regulation of the major renal sodium transporters by insulin in the kidney. Several groups, using primarily cell culture, have demonstrated that insulin can directly increase activity of the epithelial sodium channel, the sodium-phosphate cotransporter, the sodium-hydrogen exchanger type III, and Na-K-ATPase. We and others have demonstrated alterations in the expression at the protein level of many of these same proteins with insulin infusion or in hyperinsulinemic models. We also discuss how this regulation is perturbed in type I and type II diabetes mellitus. Finally, we discuss a potential role for regulation of insulin receptor signaling in the kidney in contributing to sodium balance and blood pressure. insulin resistance; ENaC; Zucker rats; streptozotocin Overview INSULIN HAS BEEN DEMONSTRATED to increase sodium reabsorption in the proximal tubule (9), the thick ascending limb (78, 102), and the distal tubule including the collecting duct (54, 144), using micropuncture and perfused tubule approaches. Over 10 years ago, Sechi (127), Butlen (30), and colleagues using I 125 -labeled insulin demonstrated binding sites in multiple cell types along the renal tubular epithelium. However, because the kidney expresses other receptors similar to the classic insulin receptor (IR), including the insulin growth factor receptor (IGF) and the IR-related receptor (IRR), the precise nature of the "receptor" that labeled insulin was binding to was not absolutely certain. Nonetheless, more recent studies showed mRNA expression of the classic IR in whole homogenates of the kidney (31, 32,130,132), and our group (150) has recently found IR expression at the protein level using immunoblotting and immunohistochemistry in the proximal tubule, thick ascending limb, and collecting duct. In this review, we discuss studies that have investigated the regulation of the major renal sodium transporters in the kidney by insulin. In addition, we examine how this regulation is altered in insulin resistance, which is a hyperinsulinemic state, as well as in type I and type II diabetes. We also discuss what is known about the regulation of the IR in the kidney, how it is affected by insulin resistance and/or diabetes, and how this might ultimately affect sodium balance and blood pressure.

. Regulation of blood pressure, the epithelial sodium channel (ENaC), and other key renal sodium transporters by chronic insulin infusion in rats.

. Regulation of the renal thiazidesensitive Na-Cl cotransporter, blood pressure, and natriuresis in obese Zucker rats treated with rosiglitazone. Am J Physiol Renal Physiol 289: F442-F450, 2005. First published April 5, 2005; doi:10.1152/ajprenal.00335.2004.-Previously, we showed an increase in protein abundance of the renal thiazide-sensitive Na-Cl cotransporter (NCC) in young, prediabetic, obese Zucker rats relative to lean age mates (Bickel CA, Verbalis JF, Knepper MA, and Ecelbarger CA. Am J Physiol Renal Physiol 281: F639 -F648, 2001). To test whether this increase correlated with increased thiazide sensitivity (NCC activity) and blood pressure, and could be modified by insulinsensitizing agents, we treated lean and obese Zucker rats (9 wk old) with either a control diet or this diet supplemented with 3 mg/kg body wt rosiglitazone (RGZ), a peroxisomal proliferator-activated receptor subtype ␥ agonist and potent insulin-sensitizing agent, for 12 wk (n ϭ 9/group). The rise in blood pressure, measured continuously by radiotelemetry, was significantly blunted in the RGZ-treated obese rats. Similarly, blood glucose and urinary albumin were markedly decreased in these rats. RGZ-treated rats whether lean or obese excreted a NaCl load faster but excreted less sodium in response to hydrochlorothiazide, applied as a novel in vivo measure of NCC activity. Obese rats had increased renal protein abundance and urinary excretion of NCC; however, this was not significantly reduced by RGZ (densitometry in cortex homogenate Ϫ %lean control): 100 Ϯ 9, 93 Ϯ 4, 124 Ϯ 9, and 141 Ϯ 14 for lean control, lean RGZ, obese control, and obese RGZ, respectively. Subcellular localization, as evaluated by confocal microscopy and immunoblotting following differential centrifugation, of NCC was not different between rat groups. Overall, RGZ reduced blood pressure and thiazide sensitivity; however, the mechanism(s) did not seem to involve a decrease in NCC protein abundance or cellular location. Decreased NCC activity may have contributed to the maintenance of normotension in RGZ-treated obese rats. insulin resistance; type II diabetes; TSC THE OBESE ZUCKER RAT is a model of gross obesity with marked insulin resistance coupled to mild hypertension. Dysregulation of sodium balance and pressure-natriuresis as a result of insulin resistance likely are major contributors to the rise in blood pressure. Previously, we (4) showed increased renal abundances of three major sodium transport proteins in the kidney of the obese Zucker rat relative to lean age mates at both 2 and 4 mo of age, i.e., the thiazide-sensitive Na-Cl cotransporter (NCC or TSC), the ␤-subunit of the epithelial sodium channel (ENaC), and the ␣ 1 -subunit of Na-K-ATPase. These relative differences in protein abundance in the kidney seemed to be attenuated some in 6-mo-old rats, as the rats became fully diabetic and the kidneys hypertrophied (3). The mechanisms underlying these changes in abundance, i.e., transcriptional vs. changes in protein turnover rate, were not determined.T...

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